1. Field of the Invention
This invention relates to a direct-injection spark-ignition internal combustion engine, and specifically, a technique for forming an air-fuel mixture.
2. Description of the Related Art
In respect of a direct-injection spark-ignition internal combustion engine in which fuel is injected directly into a combustion chamber, there is known a technique called spray guide method, in which an injector is fitted to an upper wall of the combustion chamber, in a central area thereof, and a spark plug is arranged with its electrode part located within or near a fuel injection area, so that stratified charge combustion can be carried out by directly igniting fuel spray formed from fuel injected by the injector and vaporized in part.
In such stratified charge lean combustion by the spray guide method, ignition is carried out during fuel injection period, so that combustion begins during the compression stroke in which fuel is injected. When combustion begins during or directly after fuel injection like this, an excessively rich air-fuel mixture is formed locally within the fuel spray, and such excessively rich air-fuel mixture burns at high temperatures and produces an increased amount of NOx.
Further, in the spray guide method, fuel spray gradually moves away from the spark plug, and flames generated by ignition by the spark plug spread within the combustion chamber in a manner following the movement of the fuel spray. Thus, combustion progresses relatively slowly. When combustion progresses slowly like this, the allowable limit of exhaust gas introduction quantity in EGR lowers, which makes the reduction of NOx difficult.
Slowly progressing combustion also causes a problem such that when the vaporization of fuel progresses so that the air-fuel mixture diffuses excessively, flames cannot keep up with the movement of the air-fuel mixture and vanishes so that unburned HC is emitted.
Particularly in the stratified charge combustion in low-load operation, fuel injection quantity is small and therefore injection period is short, so that the period of time in which ignition can be carried out by the spray guide method is short. Thus, there is a problem that in low-load operation, stable combustion region is narrow and stability of combustion is not sufficient.
Thus, in order to carry out stratified charge combustion stably in the direct-injection spark-ignition internal combustion engine, a technique has been developed, in which intake air flowing in through an intake port is caused to rise as tumbling flow by a cavity formed in the top face of a piston, and fuel is injected toward the side to which the tumbling flow rises (Japanese Unexamined Patent Publication No. Hei 11-210472).
In the technique disclosed in the above publication, however, the tumbling flow formed from the intake air is relatively weak. Thus, actually, it is difficult to guide the fuel spray by means of such tumbling flow, thereby stabilizing the stratified charge combustion.